The maintenance of cellular water balance is vital to all organisms. As a result, probably all cells can regulate the amount of intracellular water to an extent when challenged with either a hypoosmotic or hyperosmotic stress. This process of volume regulation usually involves the regulation of the concentration of an intracellular, osmotically active solute. Under conditions of hypoosmotic stress, water is thereby drawn into the cell and osmotic shrinkage is prevented. The solute utilized varies amongst animal species, however, most marine invertebrates utilize intracellular free amino acids. The control mechanisms of free amino acid regulation are unknown. However, our previous work has shown that when the isolated ventricle of the ribbed mussel, Modiolus demissus, is subjected to a hypoosmotic stress, cell volume regulation is accompanied by an efflux of free amino acids. The magnitude and duration of the efflux is dependent upon ATP, Ca 2 ion and Mg 2 ion concentrations. The site of action of the divalent ions is at the cell membrane surface. I have proposed a group of experiments designed to test the possibility that the control of the amino acid release rests with a membrane-bound, divalent cation requiring ATPase. Secondly, the alteration in membrane permeability may be reflected in a change in the distribution of membrane associated particles. We will examine this possibility also, utilizing freeze-fracture preparations of the red blood cells of the blood clam, Noetia ponderosa. Preliminary experiments have indicated that these blood cells volume regulate by a mechanism similar to that described for the M. demissus cardiac cells. Finally, during hyperosmotic stress the free amino acid concentration of the cardiac cells increase markedly. The source of these amino acids may be a biosynthetic pathway utilized during Anaerobic metabolism.